Helicopters and other types of aircraft can acquire a static charge during flight. This charge can be hazardous to personnel under some circumstances. Examples of such circumstances include helicopter rescue operations. In many of such operations, a helicopter approaches a person on the ground after a prolonged flight. The person being rescued (hereinafter called a “rescuee” for convenience) may be in water or otherwise not located in an area where the helicopter may land. Accordingly, it is often necessary for the rescuee to be lifted into the helicopter using a cable and hoist system. When the rescue cable is lowered to the rescuee, the static charge built up by the helicopter is discharged when the cable end (or hook or other fitting on the cable end) touches the ground or water. This charge can be significant. Moreover, the rescuee person may be hypothermic, injured or otherwise in distress, and potentially more susceptible to injury from an electrical shock.
There are various prior art systems for dissipating a static charge from an aircraft in flight. U.S. Pat. No. 4,283,749 to Buser et al. describes a grounding device attachable to an aircraft wheel housing or cargo hook, and/or which can be lowered from tree-top heights using a cable and winch. Although Buser et al. describe various embodiments, all of those embodiments require a number of conductive elements (e.g., conductive ceramic beads, conductive ceramic resistors, conductive ceramic discs) which must be assembled to form a chain of conductive elements. A simpler system could reduce manufacturing costs and be less prone to malfunction.
U.S. Pat. No. 5,208,724 to Honigsbaum describes a process and apparatus for discharging electric potential between a flying “hovercraft” (helicopter) and the ground. The Honigsbaum process uses a conductive liquid stream to establish a discharge path between the hovercraft and the earth. This system is somewhat complicated, and requires an aircraft or ground personnel to have a sufficient supply of liquid to maintain a discharge path. U.S. Pat. No. 4,059,846 to Eisenhauer teaches a lift having a grounding wire that provides a static discharge conduit between an aircraft and the ground or water. The grounding wire hangs from a metal plate used to suspend a raft from a helicopter. However, the Eisenhauer device could present a safety concern in many situations. For example, grounding wire could become entangled with trees or other ground objects if used in land rescue. U.S. Pat. No. 4,886,221 to Honigsbaum describes a charge control system using a droplet charging mechanism connected to an external portion of the aircraft. A charge control device of such a system could be damaged by debris in the air, flying objects or even animals.
There thus remains a need for simpler systems and methods to discharge electrostatic potential from a flying aircraft.